Drilling operations typically involve mounting a drill bit on the lower end of a drill pipe or “drill stem” and rotating the drill bit against the bottom of a hole to penetrate a formation, creating a borehole. A drilling fluid, typically referred to as “drilling mud”, may be circulated down through the drill pipe, out the drill bit, and back up to the surface through the annulus between the drill pipe and the borehole wall. The drilling fluid has a number of purposes, including cooling and lubricating the bit, carrying the cuttings from the hole to the surface, and exerting a hydrostatic pressure against the borehole wall to prevent the flow of fluids from the surrounding formation into the borehole.
A drilling fluid can place undesirable mechanical stress on the rock around the wellbore and may even damage the reservoir. With increasing depth a hydrostatic pressure acts outward on the borehole, which may cause mechanical damage to the formation and reduce the ability of the well to produce oil or gas. Drilling fluids also may fracture the formation, requiring a drilling shut down in order to seal the fracture. Damage to a reservoir is particularly harmful if it occurs while drilling through the “payzone,” or the zone believed to hold recoverable oil or gas.
Therefore, after a section of the wellbore has been drilled, drilling operations are stayed or ceased to seal the wellbore using a string of pipe such as casing or a liner in the well bore. The stops are commonly referred to as “casing points”. At a casing point, a sealing composition such as hydraulic cement slurry is pumped into the annular space between the walls of the well bore and the exterior of the string of pipe disposed therein. The cement slurry is permitted to set in the annular space thereby forming an annular sheath of hardened substantially impermeable cement therein. The cement sheath physically supports and positions the pipe in the well bore and bonds the pipe to the walls of the well bore whereby the undesirable migration of fluids between zones or formations penetrated by the well bore is prevented. This well-established technique has several disadvantages, including a reduction in the well diameter after each casing point and the high cost of the casing itself.
Thus, there is a continuing need for improved methods and sealing compositions for sealing subterranean zones through which fluids undesirably flow into or out of the wellbores penetrating the zones and for simultaneously increasing the mechanical strengths of the wellbore.